1. Field of the Invention
This invention relates to wireless communication systems and, more particularly, to data transmission over wireless communication systems.
2. Description of Related Art
Great strides are being made in communication systems to meet the desire to transmit data at ever-increasing speeds. Particularly, this desire is currently being addressed in wireless communication systems, such as, for example, Code Division Multiple Access (CDMA) systems.
In CDMA systems generally, base stations transmit signals to mobile terminals over a communication link referred to as a forward link, and mobile terminals transmit signals to base station over a communication link referred to as a reverse link. When a call—a communication session between a base station and a particular mobile terminal—is set up, a primary channel, referred to in some systems and herein as a fundamental channel, is set up on both a forward and reverse link. The fundamental channel can be used to transmit voice, data, and/or so-called signaling information, and transmits the signal at a particular, typically a fairly low, transmission rate. If it is desired to transmit a signal at a higher transmission rate, for example for a data transmission having a large amount of data, the CDMA system checks, as described below, to determine whether system conditions allow and it is advantageous to set up a secondary channel—a communication channel over which signals may be transmitted at the same or at a higher transmission rate than that of the fundamental channel. In some systems the secondary channel is referred to as a data channel, or, as referred to herein, as a supplemental channel. The supplemental channel is typically set up only over the particular communication link, i.e. forward or reverse, over which it is desired to transmit the signal.
If system conditions allow and it is advantageous to set up the supplemental channel, the base station sets up the supplemental channel and transmits a data burst over it. According to some current standards, a signal can be transmitted over a supplemental channel at a transmission rate of one, two, four, eight, sixteen, or thirty-two times the transmission rate of a fundamental channel, which is typically 9.6 Kbits/sec. Thus, typically, data can be transmitted on the supplemental channel at a much higher transmission rate than it could have been transmitted on the fundamental channel.
The way in which the supplemental channel is set up takes account of the fact that data is often bursty, meaning that it is transmitted in bursts followed by periods of inactivity during which no data is transmitted. Typically, the supplemental channel is maintained only for the duration of each data burst. Between data bursts, there is no supplemental channel assigned to the call, but there is a fundamental channel assigned to the call.
During the call, the mobile terminal communicates with the particular base station from which the mobile terminal receives the strongest so-called pilot signal. When the mobile terminal receives fairly strong pilot signals from more than one base station, such as for example three base stations, the mobile terminal transmits signals to all three base stations and all three base stations transmit signals to the mobile terminal. In this case, the call is said to be in soft handoff and the mobile terminal and the base stations are said to be participating in the soft handoff. The communication links between one of the base stations and the mobile terminal is typically referred to as a leg of the soft handoff. When a call is in soft handoff, there is a fundamental channel on each leg of the soft handoff, and there may also be a supplemental channel on one or more of the legs of the soft handoff. (As described above, the fundamental channel exists on both the forward and reverse links, and the supplemental channel typically exists on only the communication link over which data is to be transmitted at the higher transmission rate.) Thus, the forward link is part of at least one of the legs of the handoff, and the reverse link is part of at least one of the legs of the handoff.
Having the supplemental channel on more than one leg of the soft handoff poses several problems. One problem with having the supplemental channel on more than one leg of the soft handoff is that it uses a large amount of system resources. These resources are the various system resources, which include the number of Walsh codes—orthogonal spreading sequences—available for each of these communication link(s) that the secondary channel will use, the power level available for each of these communication links, the hardware and software resources available for each of these communication links (for example the channel elements, CPU capacity, and so-called radios), and the maximum transmission rate supportable at each of these communication links.
Using a large amount of system resources can drastically reduce the system resources available at the base stations to communicate with other mobile terminals and severely limit the number of other mobile terminals with which each of these base stations is able to communicate at a particular point in time. This drastically limits the call-handling capacity of the CDMA system.
Another problem with having the supplemental channel on more than one leg of the soft handoff is that the base stations participating in the soft handoff have to co-ordinate setting up the supplemental channel which, disadvantageously, increases the set up time of the supplemental channel.
Yet another problem with having the supplemental channel on more than one leg of the soft handoff is that the supplemental channel should have the same transmission rate on each leg of the soft handoff. Typically, this means that each base station participating in the soft handoff has to notify a central location what transmission rate it can support based on its available resources. The central location then determines the transmission rate of the supplemental channel, which is typically based on the smallest transmission rate that can be supported by any of the base stations participating in the soft handoff. This is disadvantageous in that the supplemental channel will frequently be at a lower transmission rate than can be supported by some of the base stations. Thus, the supplemental channel may have a lower data transmission rate than if it had been set up between the mobile terminal and just a particular one of these base stations that could support a higher transmission rate.
Therefore, when possible, it is beneficial to be able to have a supplemental channel on fewer than all the legs of a soft handoff. Indeed, it has been proposed that the supplemental channel should be on just one leg of the soft handoff when certain criteria are met. One of these criteria is whether the strength of the pilot signal used for the supplemental channel is changing significantly. Typically, when the strength of the pilot signal used for the supplemental channel is not changing significantly it is because the mobile terminal is moving very slowly or is stationary. In such a circumstance it may be possible for the mobile terminal to receive data at an acceptable level of signal quality over just one leg of the soft handoff. Thus, having the supplemental channel over just one leg of the soft handoff is an acceptable state of affairs. As the communication continues in soft handoff, the criteria are constantly evaluated, and if there are changes that result in whether the criteria are or are not met then the number of legs of the soft handoff on which the supplemental channel would then be set up changes.